Layers

Layers are objects on the map that consist of one or more separate items,
but are manipulated as a single unit. Layers generally reflect collections of
objects that you add on top of the map to designate a common association. The
Google Maps JavaScript API manages the presentation of objects within layers
by rendering their constituent items into one object (typically a tile overlay) and
displaying them as the map's viewport changes. Layers may also alter the presentation
layer of the map itself, slightly altering the base tiles in a fashion
consistent with the layer. Note that most layers, by design, may not be
accessed via their individual objects, but may only be manipulated as a
unit.

Layers Overview

The Google Maps JavaScript API has several types of layers:

The Google Maps Data
layer provides a container for arbitrary geospatial data.
You can use the Data layer to store your custom data, or to display GeoJSON data
on a Google map.

The Transit layer displays the public transport network
of your city on the map.

The Bicycling layer object renders a layer of bike paths
and/or bicycle-specific overlays into a common layer. This layer is returned
by default within the DirectionsRenderer when requesting
directions of travel mode BICYCLING.

Add a layer

To add a layer to a map, you only need to call setMap(),
passing it the map object on which to display the layer. Similarly, to hide a
layer, call setMap(), passing null.

The below snippet centers the map on London, UK, and adds the Transit
layer.